1. Technical Field
The present invention relates to an acceleration sensor which detects acceleration based on capacity change between a fixed electrode and a movable electrode.
2. Related Art
An acceleration sensor which detects acceleration based on capacity change between a fixed electrode provided on a support member and a movable electrode is known as an acceleration sensor. The movable electrode is provided on a movable unit having a weight, and the movable unit is connected with the support member by a spring. This type of acceleration sensor can detect acceleration having an element in the direction of capacity change between the fixed electrode and the movable electrode.
A structural resonance frequency of the acceleration sensor having this structure can be determined by mass m of the movable unit and a spring constant k with one-to-one correspondence. A value Q indicating the sharpness of the resonance is calculated by a calculation expression including an attenuation constant c as well as the mass m of the movable unit and the spring constant k, and is inversely proportional to the attenuation constant c.
For preferable transient response and shock resistance, the value Q of the acceleration sensor needs to be lowered. For example, the Q value in the range from about 0.5 to about 1.0 is required. Since the structural resonance frequency of the acceleration sensor is determined by the structure, the Q value is designed considering damping (attenuation force) indicated by the attenuation constant c as an effect produced by interaction between the components such as the support member and the movable unit and gas surrounding the components. The Q value decreases as the attenuation constant c increases.
For decreasing the Q value, a method is known which narrows the clearance between a movable member as the movable unit and a support substrate as the support member to increase a damping coefficient as the attenuation constant c utilizing slide damping produced when the movable member and the support substrate shift from each other (see JP-A-2004-286649 (pages 3 to 4, FIG. 1)).
When the clearance between the movable unit and the support member is narrow, however, collision destruction between the movable unit and the support member occurs by shock caused in the direction of narrowing the clearance between the movable unit and the support member. As a result, shock resistance lowers.